A raw material comprising a polymer compound has characteristic gas permeability for each raw material. Based on properties thereof, a desired gas component can be separated by allowing selective permeation, by means of a membrane constituted of a specific polymer compound. As an industrial application embodiment of this gas separation membrane, study has been conducted for separating and recovering carbon dioxide from a large-scale carbon dioxide source, in a thermal power station, a cement plant, a blast furnace in a steel plant or the like, in relation to a global warming issue. Then, this membrane separation technique attracts attention as a solution to an environmental issue to allow achievement by relatively small energy. Meanwhile, natural gas or bio gas (gases generated by fermentation and anaerobic digestion of excreta of organisms, organic fertilizers, biodegradable substances, polluted water, garbages, energy crops, and the like) is mainly a mixed gas of methane and carbon dioxide. Study has been made so far for a membrane separation method as a means for removing an impurity, such as carbon dioxide therein (Patent Literature 1).
In regard to the purification of natural gas according to the membrane separation method, celluloses and polyimides have been studied as the materials for the membrane for use in the membrane separation method. However, due to the high pressure conditions in actual industrial plants, the influence of impurities present in natural gas, and the like, the membrane is plasticized, and there has been a problem of a lowering in separation selectivity attributable to that (pages 313-322 of Non-Patent Literature 1, and Non-Patent Literatures 2 and 3). It has also been investigated to use polybenzoxazole (hereinafter, abbreviated to PBO), as a material which can exhibit higher durability (Patent Literature 2).
In Patent Literature 2, it is described that a membrane of PBO is obtained by forming a membrane using a particular polyimide as a precursor of PBO, and subjecting this membrane to a heat treatment at 400° C. or higher. It is also described that the resultant PBO membrane has superior permeability to carbon dioxide and gas permeation selectivity than a polyimide membrane before the heat treatment. Furthermore, it is described that as the temperature of the heat treatment is higher, permeability of carbon dioxide is enhanced.